This invention relates to heat dissipation of an integrated circuit (IC) device, in particular, to reducing the temperature gradient of an IC device. The invention also relates to heat dissipation in multi-chip modules.
As the desire for processing intensive applications increases, so does the demand for electrical systems that operate at faster speeds, occupy less space, and provide more functionality. To meet these demands, manufacturers design increasingly more powerful modules containing numerous components residing in relatively close proximity on a common substrate (e.g., an integrated circuit board). With increasing heat dissipation from microelectronics devices and reduced overall form factors, thermal management becomes increasingly important because both performance reliability and life expectancy of electronic equipment are inversely related to the component temperature of the equipment. Long life and reliable performance of a device, therefore, may be achieved by effectively controlling the device operating temperature. A typical worst-case operating temperature for a complex electronic device such as a microprocessor or an application specific integrated circuit (ASIC) device is 105xc2x0 C. (degrees Celsius).
Heat dissipation is a well-known technique for controlling operating temperatures. Heat dissipation may be accomplished in various ways such as transferring the heat to a heat-conducting medium, for example, air or liquid coolant. Due to the expense and complexity associated with active liquid cooling systems, air is typically used as the cooling medium. However, liquid cooling has the advantages of better performance, lower junction temperature and more compact size.
Dissipation of heat, either by air or liquid, often requires a series of physical interfaces to provide a thermally conductive path. These interfaces typically offer minimum resistance to heat flow and provide electrical isolation. In many applications, dissipation of the heat is aided by the use of heat spreaders and heat sinks, as shown in FIG. 1.
In FIG. 1, a heat sink 22 is attached to an integrated circuit device package 10 via an adhesive 20. Integrated circuit device package 10 contains an integrated circuit die 14 with circuitry components 12 formed thereon. A heat spreader/lid 18 is attached to integrated circuit die 14 on the opposite side of components 12 with an adhesive 16. An integrated circuit device package body 26 encloses/encapsulated integrated circuit die 14. Pins 28 provide electrical connection between integrated circuit die 14 and external circuitry (not shown). The heat generated by the operation of components 12 is dissipated to cooling medium 24 through heat spreader/lid 18, adhesives 16 and 20 and heat sink 22. For an air-moving cooling system, a fan (not shown) blows air onto heat sink 22 to transfer heat from heat sink 22 to the air in the surrounding atmosphere.
Heat sink 22 typically has substantially planar surfaces 22a and 22b and uniform thickness. In addition, traditional heat sinks are typically attached to the surface of package 10 which typically has a substantially planar surface 10a. Thus, heat sink 22 typically reduces the temperature uniformly throughout die 14. However, the various circuitry components 12 often generate different amounts of heat due to various power dissipation levels of components 12. The non-uniformity of heat dissipation causes temperature gradient/variation from one region to another on die 14. Temperature gradient is undesirable because it can result in adverse thermal gradients, causing excessive thermal stress and timing issues.
Heat sink 22 and heat spreader/lid 18 are often constructed from copper tungsten (CuW) or aluminum silicon carbide (AlSiC) which has a thermal conductivity of approximately 200 W/meterxc2x7K. Copper tungsten or aluminum silicon carbide may not provide adequate thermal conductivity for high performance integrated circuit devices. High conductivity structural materials may be created using composites which are materials having two or more different materials bonded together.
FIG. 2 shows an IC device with a composite heat dissipation structure which is composed of three layers. Specifically, composite heat dissipation structure 54 is composed of, e.g., a 0.5 mm thick lower layer 54a, a 0.5 mm thick upper layer 54c, and a 0.5 mm thick middle layer 54b sandwiched in between.
Lower layer 54a and upper layer 54c are constructed from aluminum silicon carbide (AlSiC) which provides strength and stiffness to the structure. Middle layer 54b is constructed from high grade graphite such as K1100 manufactured by Amoco which enhances heat dissipation of the structure. High grade graphite structure is typically constructed by layering high conductivity fibers, thus yields anisotropic properties which may be undesirable. For example, the high grade graphite structure has high thermal conductivity in the longitudinal direction (e.g., approximately 1600 W/mxc2x7K) but poor thermal conductivity in the vertical direction (e.g., approximately 10 to 20 W/mxc2x7K). As a result, high graphite is a good material in spreading heat along the structure in the longitudinal direction but is a poor conductor in transferring heat vertically to the surface of the heat dissipation structure.
Composite heat dissipation structure 50 also does not provide compensation for temperature gradients. As such, the temperature gradient can result in unacceptable maximum junction temperature, causing excessive thermal stress and timing issues and thereby resulting in fatigue and mechanical failure and speed limitations of the integrated circuit device.
The invention relates to methods and apparatus for dissipating heat from an integrated circuit device that minimize or eliminate temperature gradients on an integrated circuit die.
In accordance with the invention, a heat dissipating structure thermally coupled to the die has heat-dissipating characteristics tailored to match the heat-generating level of the most-proximate region of the die. In one embodiment, the heat dissipation structure may be a composite composed of a layer having various portions, each tailored to the heat-generating characteristics of the various regions of the die. For example, one portion may be formed of a material that provides relatively high isotropic conductivity and another portion may be formed of a material that provides high thermal conductivity in another direction. Because the portions of material have different heat-dissipating characteristics, they can be chosen to preserve uniformity across the integrated circuit. In one embodiment, the heat dissipation member further includes a second layer and a third layer, the first layer being sandwiched in between. The second and third layers may provide mechanical strength and stiffness for the heat dissipation member.
In one embodiment, at least one gap is formed between the integrated circuit device package and a heat dissipation member at a location corresponding to regions of various temperatures across the die. The gap increases the separation between the heat dissipation structure and the region of low-heat generation. The height of the gap may be inversely proportional to the heat generation level of the most proximate region of the die. For example, the distances of the gaps are larger at low power dissipation regions (where the IC is relatively cooler) than the distances of the gaps at high power dissipation regions (where the IC is relatively hotter). This allows the lower power dissipating regions of the integrated circuit die to increase in temperature so that they approach the temperature level of the higher power dissipating regions of the integrated circuit die, thereby normalizing the temperature of the integrated circuit die.
The gaps may be formed by shaping the mating surfaces of the heat dissipation member and/or a surface of the integrated circuit die. Alternatively, the gaps may be formed by shaping the upper surface of the package. Alternatively, the gaps may be formed by tailoring the shape of the mediate layers disposed between the heat dissipation member and the upper surface of the package. The gaps may also be formed by the insertion of additional materials at defined points between heat dissipation member and the package. Alternatively, they may be formed by shaping the surface of the die.
Other ways of tailoring the heat dissipating characteristics of the heat dissipation member, such as altering the heat-dissipating characteristics of the intervening layers, are also contemplated.
Additional heat dissipation members may be added to the structure with tailored heat-dissipating characteristics to further normalize the temperature of the integrated circuit die. The above techniques may be utilized for single-chip or multiple-chip packages.
This summary is not intended to limit the scope of the invention which is defined solely by the claims attached hereto.